Current conceptual metacommunity models predict that the consequences of local selective pressures on community structure increase with spatial isolation when species favored by local conditions also have higher dispersal rates. This appears to be the case of freshwater insects in the presence of fish. The introduction of predatory fish can produce trophic cascades in freshwater habitats because fish tend to prey upon intermediate predatory taxa, such as predatory insects, indirectly benefiting herbivores and detritivores. Similarly, spatial isolation can limit dispersal and colonization rates of predatory insects more strongly than of herbivores and detritivores, thus generating similar cascading effects. Here we tested the hypothesis that the effect of introduced predatory fish on insect community structure increases with spatial isolation by conducting a field experiment in artificial ponds that manipulated the presence/absence of fish (the redbreast tilapia) at three different distances from a source wetland. Our results showed that fish have direct negative effects on the abundance of predatory insects but probably have variable net effects on the abundance of herbivores and detritivores because the direct negative effects of predation by fish may offset indirect positive ones. Spatial isolation also resulted in indirect positive effects on the abundance of herbivores and detritivores but this effect was stronger in the absence rather than in the presence of fish so that insect communities diverged more strongly between fish and fishless ponds at higher spatial isolation. We argue that an important additional mechanism, ignored in our initial hypothesis, was that as spatial isolation increases fish predation pressure upon herbivores and detritivores increases due to the relative scarcity of predatory insects, thus dampening the positive effect that spatial isolation confers to lower trophic levels. Our results highlight the importance of considering interspecific variation in dispersal and multiple trophic levels to better understand the processes generating community and metacommunity patterns.

中文翻译：

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顶级捕食者的引入改变了空间隔离对淡水群落结构的影响

当前的概念性元群落模型预测，当受当地条件青睐的物种也具有更高的扩散率时，当地选择压力对群落结构的影响会随着空间隔离而增加。在有鱼的情况下，淡水昆虫似乎就是这种情况。掠食性鱼类的引入可以在淡水栖息地产生营养级联，因为鱼类倾向于捕食中间掠食性分类群，例如掠食性昆虫，间接使食草动物和食腐动物受益。同样，空间隔离可以比食草动物和食腐动物更强烈地限制捕食性昆虫的扩散和定植率，从而产生类似的级联效应。在这里，我们通过在人工池塘中进行现场实验来测试引入的掠食性鱼类对昆虫群落结构的影响随着空间隔离而增加的假设，该人工池塘在距源湿地三个不同距离处操纵鱼类（红胸罗非鱼）的存在/缺失。我们的研究结果表明，鱼类对捕食性昆虫的丰度有直接的负面影响，但可能对食草动物和食腐动物的丰度有不同的净影响，因为鱼类捕食的直接负面影响可能会抵消间接的积极影响。空间隔离也对食草动物和碎屑动物的丰度产生了间接的积极影响，但这种影响在没有鱼的情况下比在有鱼的情况下更强，因此昆虫群落在空间隔离较高的鱼和无鱼池塘之间差异更大。我们认为，在我们最初的假设中被忽略的一个重要的额外机制是，由于食肉动物的相对稀缺，空间隔离增加了鱼类对食草动物和食腐动物的捕食压力，从而削弱了空间隔离赋予较低营养水平的积极影响. 我们的结果强调了考虑分散和多营养水平的种间变异以更好地理解产生群落和元群落模式的过程的重要性。